Grease composition for constant velocity joint and the constant velocity joint

ABSTRACT

The invention provides a grease composition that can improve the durability and the temperature control properties of constant velocity joints and reduce the rotating torque at low temperatures, and a constant velocity joint where the above-mentioned grease composition is packed. The grease composition for constant velocity joints includes (a) a base oil containing a synthetic oil, (b) a thickener, (c) zinc dithiophosphate, (d) molybdenum dialkyl dithiocarbamate sulfide, and (e) zinc dialkyl dithiocarbamate.

TECHNICAL FIELD

The present invention relates to a grease composition capable of reducing the rotating torque of constant velocity joints at low temperatures and improving the durability and temperature control performance of the joints; and a constant velocity joint where the above-mentioned grease composition is packed. More particularly, the invention relates to a grease composition capable of improving the high-speed durability and temperature control performance of the constant velocity joints and reducing the starting rotational torque at low temperatures; and a constant velocity joint containing the above-mentioned grease composition.

BACKGROUND ART

In the current automotive industry, production of the front-engine front-wheel drive (FF) vehicles has been increasing in response to the demands to ensure a lighter weight and a larger living space. Also, the four-wheel drive (i.e., 4WD) cars are increasingly chosen for their functionality. In the FF cars and 4WD cars, the front wheels work to transmit power and control the steering. Those cars, therefore, adapt a drive shaft using a constant velocity joint capable of transmitting the rotational motion at constant speed regardless of various changes in the crossing angle formed by two axes in order to ensure smooth power transmission, for example, even when the steering wheel is turned to full lock.

The front-engine rear-wheel drive (FR) cars and the 4WD cars have a configuration that power driven by the engine is transmitted to the drive shaft of the rear wheels through a propeller shaft. It is known that the propeller shaft is a source to make noises and oscillations, and in addition transmits the noises and oscillations. The conventional propeller shaft composed of a Cardan joint and a slide spline has been replaced by a propeller shaft using the constant velocity joint capable of sliding in the axial direction as rotating at a constant velocity even though the operating angle varies.

As the performance of vehicles has been further improved in recent years, the cars producing high power are increasing. This will make a load applied to the constant velocity joint heavier and the lubricating conditions severer. At the same time, demands for a highly comfortable ride are also increasing.

The conditions where the propeller shaft is used are different from those for the drive shaft. For example, the load torque to the propeller shaft is lower, but the propeller shaft is driven under the rotation of higher speed. In light of this, the grease used in the constant velocity joint for propeller shaft is required to have more improved high-speed performance, such as superior high-speed durability and lower vibration under the high-speed operation, when compared with the grease for drive shaft.

The amount of generated heat (i.e., a rise of temperature) caused by rotation of the constant velocity joint can be used as an indicator of the high-speed performance. The limits of operating conditions of the constant velocity joint can be estimated from the above-mentioned rise of temperature.

The amount of generated heat tends to depend on the friction coefficient of grease. Development of a grease capable of exhibiting excellent temperature control performance under the circumstances of high temperature is expected.

Also, great importance is attached to smooth action of the constant velocity joint in severely cold areas. In the cold areas, the cars may probably be started under low temperature conditions. For the smooth starting of cars under such conditions, it becomes more important to reduce the rotating torque of grease at low temperatures.

However, no grease composition for constant velocity joints has been proposed that is capable of exhibiting excellent temperature control performance and durability and sufficiently reducing the rotating torque at low temperatures.

Conventionally, there are proposed grease compositions for constant velocity joints, containing a lubricant such as a base oil and a diurea based thickener, and an additive such as a molybdenum compound (as in, for example, JP 10-273691 A, JP 10-273692 A, JP 2001-11481 A, JP 2003-165988 A, JP 2005-226038 A, and JP 2006-16481 A).

There are also proposed grease compositions where a sulfur atom-containing compound is added to a grease comprising a particular trimellitic acid ester and a particular thickener (as in, for example, JP 11-131082 A).

There are also proposed grease compositions designed to stabilize the fluctuations of rotational resistance at low temperatures by using an ester oil as a base oil, and a molybdenum compound, polytetrafluoroethylene and zinc dithiophosphate as the additives (as in, for example JP 2007-138110 A).

However, the above-mentioned grease compositions for constant velocity joints are unsatisfactory in terms of the performance to control the temperature and improve the durability of the constant velocity joints when the rotational speed of the joints is as high as required of the constant velocity joints, and the performance to reduce the rotating torque at low temperatures. To make more improvements to the grease performance is needed.

SUMMARY OF INVENTION Technical Problem

An object of the invention is to provide a grease composition that can improve the durability of the constant velocity joints, suppress heat generation of the joints, and reduce the rotating torque at low temperatures, within the operating temperature range from 150 to −40° C.

Another object of the invention is to provide a constant velocity joint in which the above-mentioned grease composition is packed.

Solution to Problem

After the inventors of the invention have intensively studied to achieve the above-mentioned objects, it was found that a grease composition comprising particular ingredients can suppress heat generation of the constant velocity joints, provide the joints with sufficient durability as required, and reduce the rotating torque at low temperatures, within the operating temperature range from 150 to −40° C. The grease composition for constant velocity joints according to the invention has been thus completed based on the above-mentioned findings.

Namely, the invention provides a grease composition for constant velocity joints and the constant velocity joint, as shown below.

1. A grease composition for constant velocity joints, comprising the following components (a) to (e):

(a) a base oil including a synthetic oil,

(b) a thickener,

(c) zinc dithiophosphate,

(d) molybdenum dialkyl dithiocarbamate sulfide, and

(e) zinc dialkyl dithiocarbamate.

2. The grease composition for constant velocity joints as described in the above-mentioned item 1, characterized in that the base oil as the component (a) comprises at least one selected from the group consisting of synthetic hydrocarbon oils and synthetic ester oils.

3. The grease composition for constant velocity joints as described in the above-mentioned item 1 or 2, wherein each concentration of zinc dithiophosphate as the component (c), molybdenum dialkyl dithiocarbamate sulfide as the component (d) or zinc dialkyl dithiocarbamate as the component (e) is 0.1 to 10 mass %, based on the total mass of the grease composition.

4. The grease composition for constant velocity joints as described in any one of the above-mentioned items (1) to (3), further comprising molybdenum disulfide as the component (f).

5. A constant velocity joint where the grease composition as described in any one of the above-mentioned items (1) to (4) is packed.

ADVANTAGEOUS EFFECTS OF INVENTION

The grease composition for constant velocity joints according to the invention can improve the temperature control performance and impart the required durability to the constant velocity joints when the rotational speed is high, and reduce the rotating torque at low temperatures, within the operating temperature range from 150 to −40° C. This allows the constant velocity joint to rotate at high speed, and the vehicles to start under the circumstances of low temperatures, thereby avoiding troubles of the constant velocity joints in the severely cold area. Advantageously, the grease composition can be used for cross groove type constant velocity joints which are designed suitably for reducing the vibration caused by high-speed rotation. More advantageously, the grease composition can be used for the cross groove type constant velocity joints adapted to propeller shafts which are required to display high-speed rotation performance.

DESCRIPTION OF EMBODIMENTS

The invention will now be explained in detail.

The grease composition for constant velocity joints according to the invention is characterized by comprising the above-mentioned components (a) to (e) as essential ingredients. Those components will now be described.

The synthetic oil used for the component (a) in the invention includes generally used lubricating oils such as synthetic hydrocarbon oils, synthetic ester oils, synthetic ether oils, polyglycols and the like, and mixtures of those oils. Preferable examples of the synthetic oil include synthetic hydrocarbon oils and synthetic ester oils.

In particular, poly(α-olefin) and polybutene can be given as the preferable examples of the synthetic hydrocarbon oil for the component (a).

Preferable synthetic ester oils used for the component (a) are those prepared from alcohols preferably having 6 to 22 carbon atoms and aromatic carboxylic acids preferably having 8 to 22 carbon atoms and preferably having 2 to 6 carboxyl groups. Examples of the alcohol having 6 to 22 alcohols include 1-hexanol, 2-hexanol, 3-hexanol, 1-heptanol, 4-methyl-2-pentanol, 2-heptanol, 3-heptanol, 1-octanol, 2-octanol, 2-ethyl-1-hexanol, 1-nonanol, nonan-2-ol, 3,5,5-trimethyl-1-hexanol, 1-decanol, 1-undecanol, lauryl alcohol, myristyl alcohol, cetyl alcohol, 14-methylhexadecan-1-ol, stearyl alcohol, oleyl alcohol, 16-methyl octadecanol, icosanol, isodecyl alcohol, 18-methyl nonadecanol, 18-methyl icosanol, docosanol, 20-methyl heneicosanol, 2-octyl dodecanol and the like.

Preferably, 1-hexanol, 2-ethyl-1-butanol, 1-octanol, 1-heptanol, 2-octanol, 2-ethyl-1-hexanol, nonan-2-ol, 2-ethyl-1-octanol, isodecyl alcohol and 2-octyl dodecanol are used.

Examples of the aromatic carboxylic acids having 8 to 12 carbon atoms and 2 to 6 carboxyl groups include phthalic acid, isophthalic acid, terephthalic acid, 5-methyl isophthalic acid, 4,5-dimethoxy phthalic acid, hemimellitic acid, trimellitic acid, trimesic acid, mellophanic acid, prehnitic acid, pyromellitic acid, and mellitic acid. Preferably, 5-methyl isophthalic acid, trimellitic acid and pyromellitic acid are used.

For example, the synthetic ester oils prepared from the above-mentioned alcohols and aromatic carboxylic acids are as follows:

<Diesters> hexyl phthalic acid ester, 2-ethylbutyl phthalic acid ester, octyl phthalic acid ester, heptyl phthalic acid ester, 2-octyl phthalic acid ester, 2-ethylhexyl phthalic acid ester, nonan-2-ol phthalic acid ester, 2-ethyloctyl phthalic acid ester, nonan-2-ol isophthalic acid ester, 2-ethyloctyl isophthalic acid ester, octyl 5-methyl isophthalic acid ester, nonan-2-ol 5-methyl isophthalic acid ester, hexyl terephthalic acid ester, and octyl terephthalic acid ester. <Triesters> hexyl trimellitic acid ester, octyl trimellitic acid ester, heptyl trimellitic acid ester, 2-ethylbutyl trimellitic acid ester, 2-ethylhexyl trimellitic acid ester, nonan-2-ol trimellitic acid ester, and isodecyl alcohol trimellitic acid ester. <Tetraesters> octylbenzene tetracarboxylic acid ester, heptylbenzene tetracarboxylic acid ester, hexylbenzene tetracarboxylic acid ester, 2-ethylbutyl benzene tetracarboxylic acid ester, 2-ethyloctyl benzene tetracarboxylic acid ester, and 2-octyldodecanol pyromellitic acid ester.

Each of those esters is a full-ester where all the carboxylic acids are esterified.

The content of the synthetic oil in the base oil may be preferably 10 mass % or more, more preferably 20 mass % or more, with respect to the total mass of the base oil. The base oil may completely consist of the synthetic oils.

As the preferable example of the thickener used as the component (b) in the invention, diurea thickeners represented by the following formula (1) can be given:

R¹NH—CO—NH—C₆H₄-p-CH₂—C₆H₄-p-NH—CO—NHR²  (1)

wherein R¹ and R², which may be the same or different are each an alkyl group having 8 to 20 carbon atoms, preferably 8 to 18 carbon atoms, an aryl group having 6 to 12 carbon atoms, preferably 6 to 7 carbon atoms, or a cycloalkyl group having 6 to 12 carbon atoms, preferably 6 to 7 carbon atoms.

The diurea thickener can be obtained, for example by reacting a certain diisocyanate with a certain monoamine. Preferable specific examples of the diisocyanate include diphenylmethane-4,4′-diisocyanate. The monoamines include aliphatic amines, aromatic amines, alicyclic amines, or the mixtures thereof. Specific examples of the aliphatic amines include aliphatic amines having 8 to 18 carbon atoms such as octylamine, dodecylamine, hexadecylamine, octadecylamine and oleylamine. Specific examples of the aromatic amines include aliphatic amines having 6 to 7 carbon atoms such as aniline and p-toluidine. Specific examples of the alicyclic amines include cyclohexylamine.

As the component (b), it is preferable to use the aliphatic urea based thickeners obtainable using octylamine, dodecylamine, hexadecylamine, octadecylamine, oleylamine particularly chosen from the above-mentioned monoamines or the mixtures thereof.

The thickener as the component (b) may be contained in such an amount as to obtain a required consistency, preferably in an amount of 1 to 30 mass %, more preferably 5 to 20 mass %, based on the total mass of the grease composition.

The zinc dithiophosphate used as the component (c) in the invention includes the compounds represented by the following formula (1):

wherein R³ is an alkyl group having 1 to 24 carbon atoms or an aryl group having 6 to 30 carbon atoms, preferably an alkyl group having 1 to 5 carbon atoms.

The content of zinc dithiophosphate as the component (c) may preferably be in the range of 0.1 to 10 mass %, and more preferably 0.5 to 5 mass %, based on the total mass of the grease composition. When the content is less than 0.1 mass %, the effects will become insufficient. With the content of more than 10 mass %, further improvement in the effects cannot be recognized.

Specific examples of molybdenum dialkyl dithiocarbamate sulfide used as the component (d) in the invention may include the following compounds indicated by formula (2):

[R⁴R⁵N—CS—S]₂—Mo₂O_(m)S_(n)  (2)

wherein R⁴ and R⁵ are each independently an alkyl group having 1 to 24 carbon atoms, preferably 2 to 18 carbon atoms; and m is 0 to 3, n is 1 to 4, and m+n=4.

The content of the molybdenum dialkyl dithiocarbamate sulfide as the component (d) may preferably be in the range of 0.1 to 10 mass %, and more preferably 0.5 to 5 mass %, based on the total mass of the grease composition. When the content is less than 0.1 mass %, the effects will become insufficient. Even though the content of the component (d) exceeds 10 mass %, further enhancement of the effect cannot be recognized.

Specific examples of zinc dialkyl dithiocarbamate used as the component (e) in the invention may include the following compounds indicated by formula (3):

[R⁶ ₂N—CS—S]₂—Zn  (3)

wherein R⁶ indicates a primary or secondary alkyl group having 1 to 24 carbon atoms or an aryl group having 6 to 30 carbon atoms, preferably a primary or secondary alkyl group having 3 to 8 carbon atoms.

The content of the zinc dialkyl dithiocarbamate as the component (e) for use in the grease composition of the invention may preferably be in the range of 0.1 to 10 mass %, and more preferably 0.5 to 5 mass %, based on the total mass of the grease composition. When the content is less than 0.1 mass %, the effects will become insufficient. Even though the content of the component (e) exceeds 10 mass %, further enhancement of the effect cannot be recognized.

The mixing ratio by mass of the zinc dithiophosphate (c), the molybdenum dialkyl dithiocarbamate sulfide (d) to the zinc dialkyl dithiocarbamate (e) in the grease composition of the invention may be preferably 1: (0.3 to 3.0): (0.1 to 5.0), and more preferably 1: (0.5 to 1.5): (0.2 to 3.0).

The molybdenum disulfide optionally used in the invention as the component (f) is widely employed as a solid lubricant in the constant velocity joints. The molybdenum disulfide has a lamella lattice structure, so that its lubricating mechanism is known to reduce the frictional resistance by allowing easy sliding of the layers to cause easy shear between the layers. This component also has an effect on prevention of seizing of the constant velocity joints.

The content of the component (f) may preferably be in the range of 0.1 to 10 mass %, and more preferably 0.5 to 5 mass %, based on the total mass of the grease composition.

In addition to the above-mentioned components, the grease composition of the invention may further comprise other additives typically contained in the grease compositions, such as other extreme pressure agents, an antioxidant, a rust preventive, a corrosion inhibitor and the like.

The constant velocity joints of the invention where the member for transmitting the torque is in the form of a ball include, for example, fixed type constant velocity joints such as Rzeppa joints, bar field joints and the like; and plunging constant velocity joints such as double offset joints, cross groove joints and the like. In each of those joints, the balls, which are used as a torque transmission member are seated in tracks between an outer ring and an inner ring, and incorporated in the joint through a cage.

When the constant velocity joints of the invention are of a fixed type, the constant velocity joints include, for example, Rzeppa joints, bar field joints and the like as mentioned above. When the constant velocity joints of the invention are of a plunging type, the constant velocity joints include, for example, double offset joints, cross groove joints and the like as mentioned above, where the joints can slide in the axial direction simultaneously with the occurrence of an operating angle.

The grease composition for constant velocity joints according to the invention is applicable to any constant velocity joints with no limitation. In particular, the effects of the invention become more significant when the grease composition is used for cross groove type constant velocity joints, especially used for the cross groove type constant velocity joints for propeller shafts which are driven at higher speed rotation.

The present invention will now be explained in more detail with reference to the examples.

Preparation of Grease Compositions

After 1050 g of a base oil and 294.3 g of diphenylmethane-4,4′-diisocyanate were weighed into a first container, the resultant mixture was heated to 70 to 80° C. In a second container 460 g of a base oil and 605.7 g of octadecylamine were weighed, and the resultant mixture was heated to 70 to 80° C. and then added to the first container. With thoroughly stirring, the reaction was caused for 30 minutes. After stirring under application of heat, the reaction mixture was cooled, thereby obtaining a urea grease base product. To the urea grease base product thus obtained, additives were added in such amounts as shown in Table 1, and the base oil was appropriately added to obtain a mixture. Using a three-roll mill, the resultant mixture was adjusted to have a grade 1 in terms of consistency.

The characteristics of the grease compositions obtained in Examples and Comparative Examples were evaluated in accordance with the following test methods. The results are shown in Table 1.

(1) Kinematic Viscosity of Base Oil

The kinematic viscosity of base oil was determined at 100° C. in accordance with JIS K2283.

(2) Low-Temperature Torque Test

The starting torque at −40° C. was measured in accordance with JIS K2220.

The evaluation criteria are as follows:

Starting torque: 1500 mN·m or less good=o

-   -   more than 1500 mN·m not good=x

(3) SRV Test

The test was conducted in accordance with ASTM D5707.

Test pieces ball with a diameter of 17 mm (SUJ2)

-   -   plate: 24 mm (dia.)×7.85 mm (SUJ2)

Test conditions

-   -   Load: 500 N     -   Frequency: 50 Hz     -   Stroke amplitude: 1.5 mm     -   Duration: 60 minutes     -   Test temperature: 150° C.     -   Measuring items: Coefficient of friction, diameter of wear scar         on ball

The evaluation criteria are as follows:

SRV test

-   -   Coefficient of friction: 0.4 or less good=o         -   more than 0.4 not good=x     -   Diameter of wear scar: 0.7 mm or less good=o         -   more than 0.7 mm not good=x

(4) Bench Test

Test conditions

-   -   Number of revolutions: 6000 rpm     -   Torque: 200 N·m.     -   Angle: 3 deg.     -   Operating time: 100 hours     -   Type of joint: cross groove joint

Measuring items:

-   -   Temperature control properties: The temperature of the surface         of the outer ring was measured during the test.

The evaluation criteria are as follows:

-   -   120° C. or less good=o     -   more than 120° C. not good=x     -   Durability: The area where the flaking took place on a portion         of the joint was determined after the test.         The evaluation criteria are as follows:

Flaking occurrence area on outer ring groove: less than 20 mm² excellent=00

-   -   Flaking occurrence area on outer ring groove: less than 25 mm²         good=o     -   Flaking occurrence area on outer ring groove: 25 mm² or more not         good=x

TABLE 1 Examples Comparative Examples 1 2 1 2 3 (a) Base oil 79 78.5  79 80 80 Mineral oil 70 70 100 70 70 Synthetic oil 30 30 — 30 30 (b) Diurea based thickener 18 18  18 18 18 (c) ZnDTP*¹ 1 1  1 — 1 (d) MoDTC*² 1 1  1 1 — (e) ZnDTC*³ 1 1  1 1 1 (f) MoS₂*⁴ — 0.5 — — — (g) S-containing extreme — — — — — pressure agent*⁵ (h) Pb-containing extreme — — — — — pressure agent*⁶ (i) Fatty acid amide — — — — — (j) S—N based extreme — — — — — pressure agent*⁷ (k) Polytetrafluoroethylene — — — — — Kinematic viscosity 13.6 13.6   13.2 13.6 13.6 of base oil at 100° C. (mm²/s) Torque at low Starting torque 1300 1300 1500< 1300 1300 temperatures ◯ ◯ X ◯ ◯ SRV test Diameter 0.66 0.64    0.67 0.80 0.83 of wear scar ◯ ◯ ◯ X X Coefficient 0.04 0.04    0.04 0.07 0.11 of friction ◯ ◯ ◯ X X Bench test Temperature 100 100 100 140 150 control properties ◯ ◯ ◯ X X Durability 22 16  21 35 32 ◯ ◯◯ ◯ X X Comparative Examples 4 5 6 7 8 9 (a) Base oil 80 79 78.5 78.5 77.5 77 Mineral oil 70 70 70 70 70 70 Synthetic oil 30 30 30 30 30 30 (b) Diurea based thickener 18 18 18 18 18 18 (c) ZnDTP*¹ 1 1 1 1 — 1 (d) MoDTC*² 1 1 1 1 1 1 (e) ZnDTC*³ — — — — — — (f) MoS₂*⁴ — — 0.5 0.5 2 2 (g) S-containing extreme — — 1 — 1 — pressure agent*⁵ (h) Pb-containing extreme — 1 — — — — pressure agent*⁶ (i) Fatty acid amide — — — 1 — — (j) S—N based extreme — — — — 0.5 — pressure agent*⁷ (k) Polytetrafluoroethylene — — — — — 1 Kinematic viscosity 13.6 13.6 13.6 13.6 13.6 13.6 of base oil at 100° C. (mm²/s) Torque at low Starting torque 1300 1300 1300 1300 1300 1300 temperatures ◯ ◯ ◯ ◯ ◯ ◯ SRV test Diameter 0.74 0.78 0.73 0.76 0.80 0.79 of wear scar X X X X X X Coefficient 0.04 0.04 0.04 0.04 0.11 0.04 of friction ◯ ◯ ◯ ◯ X ◯ Bench test Temperature 110 120 110 120 150 110 control properties ◯ ◯ ◯ ◯ X ◯ Durability 29 27 28 30 32 30 X X X X X X *¹⁾Zinc dithiophosphate *²⁾Molybdenum dialkyl dithiocarbamate sulfide *³⁾Zinc dialkyl dithiocarbamate *⁴⁾Molybdenum disulfide *⁵⁾Sulfur-containing extreme pressure agent (Content of S: 43%) *⁶⁾Lead-containing extreme pressure agent (Content of Pb: 15%) *⁷⁾Sulfur-nitrogen extreme pressure agent

As can be seen from the above, within a range of operating temperatures from 150 to −40° C., the grease compositions for constant velocity joints obtained in Examples 1 and 2 according to the invention can reduce the rotating torque at low temperatures and exhibit superior durability and temperature control properties under the conditions of high-speed rotation when compared with the grease compositions obtained in Comparative Examples 1 to 4 where any one of the components (a), (c), (d) and (e) is not contained. The grease composition of Example 2 comprising the component (f) is found to be superior to that of Example 1 free from the component (f) in terms of the durability under the conditions of high-speed rotation.

The wear scar on the ball becomes larger in the SRV test, indicating poor durability in Comparative Example 5 where the Pb-containing extreme pressure agent is used as the component (h) instead of the component (e), i.e., ZnDTC as in Example 1; Comparative Example 6 (corresponding to JP 10-273691 A) where the S-containing extreme pressure agent is used as the component (g) instead of the component (e), i.e., ZnDTC as in Example 2; Comparative Example 7 (corresponding to JP 2001-11481 A) where the fatty acid amide is used as the component (i) instead of the component (e), i.e., ZnDTC as in Example 2; and Comparative Example 9 (corresponding to JP 2007-138110 A) where MoS₃ of the component (f) and polytetrafluoroethylene of the component (k) are used instead of the component (e), i.e., ZnDTC as in Example 2.

In Comparative Example 8 (corresponding to JP 10-273692 A) where MoS₃ of the component (f), the S-containing extreme pressure agent of the component (g) and the S—N based extreme pressure agent of the component (j) are used instead of the component (c), i.e., ZnDTP and the component (e), i.e., ZnDTC as in Example 1, the coefficient of friction becomes higher and the wear scar on the ball becomes larger in diameter in the SRV test and the temperature control properties and the durability are also worsened. 

1.-5. (canceled)
 6. A grease composition for constant velocity joints, comprising the following components (a) to (e): (a) a base oil including a synthetic oil, (b) a thickener, (c) zinc dithiophosphate, (d) molybdenum dialkyl dithiocarbamate sulfide, and (e) zinc dialkyl dithiocarbamate.
 7. The grease composition for constant velocity joints of claim 6, wherein the synthetic oil in the component (a) comprises at least one selected from the group consisting of synthetic hydrocarbon oils and synthetic ester oils.
 8. The grease composition for constant velocity joints of claim 6, wherein the content of the zinc dithiophosphate as the component (c), the content of the molybdenum dialkyl dithiocarbamate sulfide as the component (d) and the content of the zinc dialkyl dithiocarbamate as the component (e) are each in the range of 0.1 to 10 mass % of the total mass of the grease composition.
 9. The grease composition for constant velocity joints of claim 7, wherein the content of the zinc dithiophosphate as the component (c), the content of the molybdenum dialkyl dithiocarbamate sulfide as the component (d) and the content of the zinc dialkyl dithiocarbamate as the component (e) are each in the range of 0.1 to 10 mass % of the total mass of the grease composition.
 10. The grease composition for constant velocity joints of claim 6, further comprising molybdenum disulfide as a component (f).
 11. A constant velocity joint wherein the grease composition of claim 6 is packed. 